Vehicles such as aircraft may use an alternating current (AC) synchronous dynamoelectric machine as a generator powered by a prime mover, typically a gas turbine engine. Typically, a separate air turbine starter for large engines or a separate dynamoelectric machine for small turbines, usually a direct current (DC) dynamoelectric machine, serves as a starter dynamoelectric machine 6. It is more desirable to use the AC synchronous dynamoelectric machine as both a generator and a starter dynamoelectric machine 6 to eliminate the weight, bulk and mechanical complexity of a separate air turbine or electric starter dynamoelectric machine 6 and associated clutch for disengaging the starter dynamoelectric machine 6 after a start operation. A single AC synchronous dynamoelectric machine may provide savings in weight, space and mechanical complexity compared to separate machines for starter dynamoelectric machine 6 and generator functions in combination with a mechanical clutch mechanism for disengaging the starter dynamoelectric machine 6 after the start operation.
It is essential to know the precise position of a rotor or shaft for a synchronous dynamoelectric machine at all times during a starting process. To enhance reliability and reduce weight it is desirable to have a “sensorless” electric starting system for the dynamoelectric machine. Sensorless starting implies that there is no dedicated angular position or velocity sensor associated with the dynamoelectric machine.
Although there are many different sensorless control algorithms that are suitable for such a sensorless starting system, there presently is no practical sensorless method that works from the maximum normal operating speed all the way down to zero speed. Because starting begins at zero speed, it is necessary to have a work-around for this limitation. In fact, a method that is acceptable, reasonably robust, and most used is a method that relies on slow “open loop” acceleration up to about 10 percent of normal speed. Beyond 10 percent speed system control transfers to a closed loop algorithm that uses the electrical potential and current of the dynamoelectric machine to derive shaft position and velocity. At 10 percent speed and above, there is generally sufficient electromotive force (EMF) available on from the dynamoelectric machine to provide the needed angular position information. Below 10 percent speed, the EMF of the dynamoelectric machine is generally so small that it is difficult to accurately determine angular position even with algorithms that correct for the electrical potential drop across the internal impedance of the dynamoelectric machine.
In the range of zero to 10 percent speed, the difficulty and lack of robustness associated with all sensorless closed loop control algorithms make it almost impossible to adequately determine shaft position whilst simultaneously delivering torque-producing current to the dynamoelectric machine for engine starting. Accordingly, the starting system uses the open loop slow acceleration method for starts beginning at zero speed with a transition to sensorless closed loop control at a sufficiently high speed, such as approximately 10 percent, to assure a continuous and smooth starting torque over the entire starting speed range.
As the name implies, open loop control means that the electrical potential applied to the dynamoelectric machine is independent of rotor position. During open loop control, the starting system controls the applied electrical potential, current and frequency to provide a rotational torque and accelerating speed for the dynamoelectric machine. It is desirable and assumed that the rotor of the dynamoelectric machine follows this applied electrical potential, current and frequency acceleration profile. Typically, a selected acceleration profile must be slow enough to provide adequate torque margin between the maximum available from the dynamoelectric machine (pullout torque) and the load on the dynamoelectric machine due to engine drag and inertial torque. Because the rotor position remains unknown for the open loop operation, only when the acceleration and speed of the rotor are at or close to zero it is generally acceptable to engage the open loop part of the start sequence.
However, there are many instances where it is necessary to restart the dynamoelectric machine at other than zero speed. Examples of such instances that may require “re-engagement” of the starting process between zero speed and closed loop speed are aborted start attempts, engine coasting and engine windmilling, such as occurs with an engine mounted aboard an aircraft. It may be unacceptable to wait a during a pre-determined period that assures that speed has reached zero before beginning an engine start from standstill, especially when the engine is needed for emergency situations or for safe operations. Because re-engagement may be required for any speed between zero speed and cut out speed of the dynamoelectric machine in its starting mode, it is essential to provide that function for sensorless starters including open loop speeds where the sensorless algorithm is not functional.